Method for producing a minimum-ridging type 430 stainless steel

ABSTRACT

A PROCESS FOR PRODUCING A TYPE 430 STAINLESS STEEL SHEET PRODUCT WITH SUBSTANTIALLY REDUCED RIDGING CHARACTERISTICS WHEN DRAWN. COMMENCING IN SLAB FORM, THE TYPE 430 STEEL IS HOT ROLLED AT 1850 TO 1950* F. TO A THICKNESS OF 0.1 TO 0.25 INCH; BOX ANNEALED AT 1750 TO 1900* F. FOR AT LEAST ONE HOUR AND THEN AT 1450 TO 1500* F. FOR AT LEAST TWO HOURS; COLD ROLLED WITH A THICKNESS REDUCTION OF FROM 35 TO 75%; AND CONTINUOUS ANNEALED AT FROM 1400 TO 1500% F.

United States Patent O 3,684,589 METHOD FOR PRODUCING A MINIMUM- RIDGING TYPE 430 STAINLESS STEEL Kenneth G. Brickner, OHara Township, Allegheny County, and George A. Ratz, Bethel Park Borough,

Pa., assignors to United States Steel Corporation No Drawing. Filed Oct. 2, 1970, Ser. No. 77,679

Int. Cl. CZld 9/46, 9/48 US. Cl. 148-12 2 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION It is Well known that the 17% chromium ferritic stainless sheet steels (i.e., AISI Type 430 stainless steels) usually exhibit a surface defect known as ridging or roping when subjected to severe cold forming operations such as deep drawing. This defect appears as narrow raised areas similar to corrugations parallel to the cold rolling direction. Ridging is of course detrimental to the appearance of the drawn part, and expensive grinding and polishing operations are required to suitably improve the appearance.

Although many processes and procedures have been developed to overcome the ridging problems associated with the AISI Type 430 stainless steels, these processes have not been altogether satisfactory, at least from a commercial point of view. According to one process for example (US. Pat. No. 2,965,479), ridging can be eliminated by adding small amounts of columbium to the alloy. Although this process is used commercially to a limited extent, it greatly adds to the cost of the product, not only because of the cost of the columbium itself, but because the columbium causes processing ditficulties resulting in lower yields. Most other procedures for reducing the ridging problem have involved the careful control of processing parameters, i.e., rolling and annealing, to eflFect particular microstructures allegedly found to be non-ridging in nature. Of particular significance to this invention is a process disclosed in US. Pat. No. 2,851,384, Waxweiler, Sept. 9, 1958. According to this patent, ridging is minimized by increasing the amount of austenite to above 35% at the hot rolling temperatures, and then subjecting the steel to carefully controlled heat treatments to randomize the texture or orientation of the hot rolled grains. The austenite content is controlled through close control of carbon, nitrogen, silicon, nickel and manganese in a 14-18% chromium stainless steel. Although this process, and other prior art proceses, are effective to some extent in minimizing ridging in conventional Type 430 stainless steels, none of the prior art processes have been completely successful in eliminating the ridging problem. Hence, there still remains considerable area for improvement.

SUMMARY OF THE INVENTION An object of this invention is to provide a new and improved process for providing AISI Type 430 stainless steel sheet and strip :which is more effective than the prior art processes in minimizing the ridging problems associated therewith.

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Another object of this invention is to provide a process for producing Type 430 stainless steel sheet and strip having substantially reduced ridging characteristics when subjected to severe cold forming operations, and further having tensile properties equivalent or superior to those of prior art Type 430 stainless steel sheet and strip.

A further object of this invention is to provide Type 430 stainless steel sheet and strip which can be drawn with minimum ridging in the strained portion thereof.

These and other objects and advantages will become apparent from the following detailed description of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The process of this invention is applicable to the A151 Type 430 stainless steels which have a nominal composition as follows:

Percent Carbon max 0.15 Manganese max 1.0 Silicon max 1 .0 Chromium 16-18 Iron, balance.

Besides the above listed elements, the alloy may of course contain the usual steelmaking impurities in amounts insufiicient to affect the alloys properties.

Stated briefly, the process of this invention, whereby a Type 430 stainless steel is produced with minimum ridging characteristics, involves the careful control of all processing parameters in producing the sheet from slab form. Of critical significance are the hot rolling temperatures, the box annealing times and temperatures following hot rolling, the cold reduction, and the continuous annealing temperatures following cold reduction.

More specifically, the preferred process of this invention comprises (1) hot rolling of a slab of Type 430 stainless steel between 1850 and 1950 F. to a thickness of from 0.1 to 0.25 inch; (2) box annealing the hot rolled product at a temperature between 1750 to 1900 F. for at least one hour, and then continuing the box anneal for at least two hours at a reduced temperature of from 14 50 to 1500 F and then furnace cooling the annealed product; 3) pickling and cold rolling the annealed product with a cold reduction of from 35 to and (4) finally continuous annealing the cold rolled product at a temperature of from 1400 to 1500 F. and pickling the cold rolled and annealed sheet or strip product. Steps (3) and (4) may be repeated if necessary in order to attain a product thinner than can be achieved with a. 7 5% cold reduction.

Although the details of the microstructural development and the cross relationships between the various parameters are not completely understood, we have found that each of the above specified parameters is critical if an improved minimum-ridging product is to be achieved.

In the manufacture of Type 430 stainless steel sheet and strip by prior art practices, conventional slab hot rolling temperatures of about 2100 to 2200" F. are utilized. In the practice of this invention, however, lower slab hot rolling temperatures of from 1850 to 1950 F. are critical to achieve the substantially increased ridging resistance which characterizes the invention. When this rolling temperature range is used, the finishing temperature of the hot rolling operation is not critical nor are the individual hot reductions. As in prior art practices, therefore, we prefer to hot roll sufficient to produce a hot band of from 0.1 to 0.25 inch thick.

In a like manner, this inventive process requires lower, critical box annealing temperatures as compared to prior art practices. Whereas box annealing temperatures are usually in the subcritical range, i.e., 1450-1500 F., prior art literature has taught that in order to minimize ridging in a Type 430 steel, higher box annealing temperatures of about 1950" F. are necessary. Contrary to both prior art practices, this process requires a critical annealing temperature not greater than 1900" F., and preferably within the range 1750 to 1900 F., for at least one hour, and then an even lower critical temperature of from 1450 to 1500 F. for at least two more hours. During the first hour of box annealing within the range 1750 to 1900 F., temperatures in the low end of the range are preferred in order to minimize grain coarsening. Such grain coarsening frequently causes an orange peel effect on the surface of deep drawn articles, which, like ridging, is undesirable insofar as appearance is concerned. It is further preferred that the box anneal be performed in a non-oxidizing, non-nitrogen containing atmosphere, such as hydrogen, in order to minimize annealing border, i.e., a heavily oxidized outer edge on the annealed coil.

The dual temperature feature of the box anneal, as described above, is primarily to prevent an undesirable open surface characteristic. That is to say, the pickling to remove surface scale following the box anneal has a tendency of producing an open or pitted surface. We believe this is caused by the pickling solution dissolving surface metal adjacent to precipitated carbides at the grain boundaries, which are formed during the box anneal. This detrimental open surface can be markedly reduced by lowering the temperature of the annealing furnace after at least one hour as described. This low temperature treatment reduces the susceptibility of the grain boundaries to attack by the pickling solution. If preferred, the metal can be completely cooled to ambient temperatures between the 1750-1900 F. anneal and the 1450-4500 F. anneal without adverse effect.

As stated above, the cold rolling procedure following the box anneal should reduce the annealed hot band from about 35 to about 75%. When the hot band is produced in accordance with conventional prior art practices, the extent of cold reduction seems to have little or no effect on ridging characteristics. However, in the practice of this invention, and for reasons we cannot explain, wherein the critical hot rolling temperatures are followed and the critical box anneal parameters maintained as described, total cold reductions of from 35 to 75% are critical to achieve a substantially greater degree of ridging resistance. In a like manner, the hot rolling temperatures and the box annealing times and temperatures do not individually have a significant eflect on the steels ridging behavior unless both of the other operations are practiced within the critical ranges. Hence there is an unexplainable interaction between hot rolling temperatures, annealing times and temperatures and the degree of cold rolling which, when properly combined, will yield a Type 430 stainless steel sheet or strip having a substantially greater resistance to ridging than is possible with prior art practices.

As stated above, the final anneal, and any other intermediate anneal during cold rolling should be a continuous anneal at a temperature within the range 1400 to 1500 F. This anneal is a simple stress relief anneal below the transformation temperature, and is substantially the same as has been the practice in prior art processes and, therefore, needs no further discussion.

The pickling procedures following the box anneal and the continuous anneal are of course merely to remove surface scale and may, therefore, conform to any prior art pickling process.

The following examples of tests and the results thereof TABLE I Chemical compositions of the steels investigated (percent) Steel 0 Mn P 8 Si Cr Ni Cu Mo N From these two heats many test specimens were prepared and processed at differing parameters to ascertain the varying elfects of the differing parameters. The variables studied in detail were (1) initial slab rolling temperature; (2) finishing temperature of slab hot rolling; (3) box annealing temperature; (4) boX annealing time; (5) method of cooling after box annealing; (6) and amount of cold reduction.

For testing purposes, specimens 1 /2 inch wide by 14 inches long were cut from the finished product in the longitudinal direction and then polished to a mirror finish. All specimens were then strained 15% in tension at room temperature. Specimens were visually examined and rated for severity of ridging according to the following arbitrary scale: 0none; 1very slight; 2slight; 3-moderate; 4severe; and 5-very severe.

Although the detailed data on all the tests conducted and the results obtained is too voluminous to be presented here, it can be said that those specimens processed within the limits of this invention had an average ridging rating of 0.8. Other specimens wherein one or more of the processing parameters fell outside the scope of this invention had average ridging ratings of about 2.2.

We claim:

1. A process for producing a Type 430 stainless steel sheet product with substantially reduced ridging characteristics when drawn, comprising forming a steel slab having the composition by weight percent:

Percent carbon up to 0.15 manganese up to 1.0 silicon up to 1.0 chromium 16 to 18 iron, balance except for incidental impurities;

hot rolling said slab at a temperature of from 1850 to 1950 F. to a thickness of from 0.1 to 0.25 inch; box annealing the hot rolled steel at a first annealing temperature of from 1750 to 1900 F. for at least one hour, and then at a second annealing temperature of from 1450 to 1500 F. for at least two hours; cold rolling the annealed steel to final thickness with a reduction of from 35 to continuous annealing the cold rolled steel at from 1400 to 1500 F.; and cooling the steel product.

2. The process of claim 1 in which the cold rolling and continuous annealing steps are repeated.

References Cited UNITED STATES PATENTS 2,808,353 10/1957 Leffingwell et al. l48-l2 3,128,211 4/1964 Waxweiler 148-12 3,139,358 6/1964 Graziano 14812 3,141,800 7/196'4 Reichenbach 14812 3,490,956 1/1970 Wilton 148l2 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner 

